A passive optical network (PON) is a system that brings optical fiber cabling and signals all or most of the way to the end user. The passive simply describes the fact that optical transmission has no power requirements or active electronic parts once the signal is going through the network.
A traditional PON consists of an Optical Line Termination (OLT) at the communication company's office and a number of Optical Network Units (ONUs) near end users. An ONU is a form of Access Node that converts optical signals transmitted via fiber to electrical signals that can be transmitted via coaxial cable or twisted pair copper wiring to individual subscribers.
The OLT services a number of ONUs typically connected in a star arrangement using optical splitters, which typically reside at a premise of a user. Multiple ONT units share a single upstream wavelength to communicate with the OLT at the head end. Each ONT unit assigns a specific time slot in which to communicate. This is an optical multipoint-to-point bus that is subject to corruption if an ONT unit transmits at any time other than during its assigned time slot. When this occurs, the output collides optically with each other causing a collision.
Traditional methods to detect collisions on multipoint, shared optical media require deducing an insubordinate ONT unit is present from error indications on the PON. These error indications are the same ones used to monitor the general function of an ONT unit. There is insufficient resolution in these error indications to identify this type of error. For example, it is possible that a loss of burst is caused by cyclic redundancy check (CRC) mismatch, bit interleaved parity (BIP) error, or header corruption. Although an insubordinate ONT unit could cause these indications, other impairments or functional issues can also cause them. General responses to an ONT unit exhibiting these behaviors are to disable the ONT unit in question. If the failure is caused by an insubordinate ONT unit, the wrong unit may have been disabled.
Current collision solutions depend on inference of the collisions at a higher logical layer from the lost of burst, error rates, or other indirect means. The solutions do not provide conclusive deductions of the existence of the collisions because other types of failures or degradation can generate the problem. Failure signature of a collision at the media access control (MAC) layer suggests an ONT unit that is faulty, with the possible result that the affected (disturbed) unit is disabled. Consequences of this action are complete loss of service to the associated customer. Therefore, a more reliable means of detection is desired to prevent the disabling of a functioning ONT unit, and facilitate detection of a faulty ONT unit.